1. Field of the Invention
The invention relates to a method and device for improving the printing quality and performance of electrographic printers, in which a latent electrical charge pattern of electric signals is produced by means of an electrode matrix or the like, which temporarily produces electrical fields for attraction of pigment particles towards an information carrier.
2. Description of the Related Art
International Patent Application PCT/SE88/00653 discloses a method for developing pictures and text with pigment particles on an information carrier directly from computer-generated electric signals, without the need for these signals to be intermediately stored for a temporary conversion to light energy, which is necessary for photoconductive printers, e.g., laser printers. These problems have been solved by bringing the information carrier into electrical cooperation with at least a screen or preferably a lattice-shaped electrode matrix which through control in accordance with the configuration of the desired pattern, at least partly opens and closes passages through the matrix, which is galvanically connected to at least one voltage source. An electrical field is exposed through the open passages for attraction of the pigment particles towards the information carrier.
This method, herein referred to as the EMS concept, and described in the above-mentioned patent application, may result in printing which does not have a high enough quality, especially with repeated and continuous use.
A problem which can occur with repeated and continuous use of devices according to the EMS concept is that electrodes of the electrode matrix gradually become covered with toner. This gradual covering in itself does not necessarily bring about any degradation of the quality of the print, but under certain circumstances the print may degenerate when certain meshes are blocked or when excess toner quantities are deposited on the paper, whereby the print becomes uneven and bleached, the definition disappears, and the blackness of the print becomes too high.
Another problem which occurs with the development in the EMS concept concerns the magnetic field which normally is required for the transport of toner from its container to the immediate proximity of the electrode matrix. The most common method when using magnetic toners is to let the roller which transports the toner, herein referred to as the developing roller, surround a core having several magnetic poles, the magnetic flow of which is substantially directed orthogonal to the longitudinal axis of the roller. Since commercially available developers are almost exclusively used in photoconductive printers and copiers, the poles of the magnetic cores have been dimensioned and oriented such that the development of toner shall be as favorable as possible during a short path along the periphery of the envelope surface of the developing roller. Several embodiments of the EMS concept are based on multiple line electrode matrices where development must be possible along a longer path of the periphery of the developing roller than in the case of photoconductive processes. The extension of the electrode matrix in certain cases may demand a path ten times longer. When using the design of magnetic poles which is common today, the printing in the mesh lines which are situated furthest from the center of the magnetic pole, in which proximity development is intended to take place, will be weaker or not appear at all as a result of a non-optimal magnetic field pattern in these areas. Further, the curvature of the envelope surface of the developing roller often cooperates with these non-optimal magnetic forces in an unfavorable way, so that the conditions for good printing quality further are degraded at the outermost lines of the electrode matrix.
These above-described problems are not limited to the EMS concept, but are also present, wholly or partially, in several electrographic printer concepts, where passage of the toner is created in an electrical manner.
Common to all problems described herein, another drawback of the known technique is that the printing quality, and thereby the readability, is influenced negatively, resulting in reduced competitiveness and lower consumer value.